Because a fingerprint including a large number of striped pattern-like ridges is said to include two major features such that it is invariable lifelong and unique to each individual, it has been used as a means to identify individuals for many years.
In a fingerprint examination, that is, a difference judgment of fingerprints for judging whether two fingerprints are of the identical finger, an examiner had compared two fingerprint images with the eye, and judged whether feature points existed at corresponding positions on ridges of both fingerprints by visual observation. And when the number of the feature points which formed a pair in the two fingerprints, that is, the feature points which existed at corresponding positions on the ridges of both of the fingerprints, was more than a given number, the examiner had judged that those two fingerprints were from the identical finger. Further, a ridge is a line-like bulge which exists on a skin of a finger or a palm, or a stripe-like pattern of a fingerprint or a palmprint impressed by the line-like bulge.
In a fingerprint image or a palmprint image, a feature point often indicates only an ending point or a bifurcation point of a ridge which forms a stripe. Also, feature points which form a pair on two images (in this case, fingerprint images) are paired feature points.
In a trial, together with two of fingerprint pictures or fingerprint gray images which are placed side by side, material which shows the identified relation between paired feature points is submitted. An example of the material mentioned above submitted in a trial is disclosed in a non-patent document 1.
In recent years, a fingerprint matching system using a computer is spreading, and accordingly, evidentiary material for trials is also often created using a computer. Generally speaking, charting is to display two fingerprint images side by side, to connect paired feature points of the two fingerprint images by a line, and to enter a number assigned to the paired feature points. Also, in a system for creating material for trials by a computer, a function to display two fingerprints and to support a manual input or modification of paired feature points is a charting function (Charting Function).
A computer equipped with the charting function often displays two fingerprints, which are targets of a difference judgment, placed side by side. Such a display method is side by side display. A figure or a screen with the side by side display is also called a charting diagram or a charting screen. Also, two corresponding points are charting points. A line which connects two charting points which are often displayed on the charting screen is a charting line.
When identifying a charting point, an examiner usually counts by visual observation the number of ridges that exist between the target charting point and a charting point in the neighborhood on two fingerprint images which are included in a judgment target. In case the counted number of crossing ridges matched between two fingerprints of the judgment target, the examiner judged that the identified charting point was correct.
An example of a feature point display device which performs charting automatically is disclosed in Patent document 1.
The feature point display device of Patent document 1 includes a fingerprint information input unit, a display unit, a calculation unit, a numbering unit, a determination unit, a decision unit, a control unit and a detection unit, and operates roughly as follows.
The fingerprint information input unit finds a core of a fingerprint from two inputted fingerprint images. The calculation unit detects feature points from each fingerprint image and calculates distances between the detected feature points and the core of the fingerprint. The numbering unit assigns a number to each of the detected feature points. The determination unit calculates, in numerical order of the number assigned to the feature point, between each of the feature points of one fingerprint image and each of the feature points of other fingerprint image, a degree of certainty which is calculated based on a position and so on of each of the feature points and which indicates a degree that two feature points match. The decision unit extracts each feature point pair whose degree of certainty exceeds a predetermined threshold value, and, in order of the number that is assigned to the feature point detected in one fingerprint, assigns a number to each of the extracted feature point pairs. The detection unit deletes each feature point pair designated by user's operation. The control unit makes the display unit display each of the feature point pairs which are assigned the number by the decision unit and which are not deleted by the decision unit. The display unit displays the two fingerprint images side by side, and further displays each of the feature point pairs in the fingerprints, each line which connects the feature points which form a feature point pair, and the number assigned to each of the feature point pairs, by overlaying them on the fingerprint image.
In Patent document 2, an example of a fingerprint correction display method is disclosed which displays two fingerprint images by a fingerprint display device after correcting one fingerprint image based on the other fingerprint image.
Correction method of Patent document 2 includes a process that detects corresponding feature points in the two fingerprint images, a process that calculates a distance between the detected feature point and a fingerprint core, a process that calculates a ratio of the distances each of which is the distance between the corresponding feature point and the fingerprint core, and a process that corrects one fingerprint image according to the calculated ratio of the distances. The correction method of the fingerprint display device of Patent document 2 is roughly as follows.
The fingerprint display device first detects feature points from the two fingerprint images respectively. Next, the fingerprint display device detects feature points which corresponds each other, among the feature points detected on each of the fingerprint images. The fingerprint display device calculates the distance between each of the detected corresponding feature points and the fingerprint core, for each of the fingerprints, and calculates the ratio of the distances. The fingerprint display device further performs correction which transforms, using one fingerprint image as a standard, other fingerprint image according to the calculated ratio of the distances.
An example of a striped pattern image examination device which displays feature data of two fingerprint images for which a difference judgment is performed is disclosed in Patent document 3.
The striped pattern examination device of Patent document 3 includes a fingerprint feature data extraction means, a feature point data matching means, a skeleton data matching means, a correspondence relationship modification means, an image distortion modification means, and an image displaying means and operates roughly as follows.
The fingerprint feature data extraction means extracts, from the two fingerprint images which are to be examined, a skeleton which is a thinned ridge of the fingerprint image, and extracts a feature point (ending point and bifurcation point) of the fingerprint. The feature point data matching means corresponds feature points which form a pair on the two fingerprint images and extracts all the pairs of the feature points.
The skeleton data matching means corresponds skeleton points extracted from the two fingerprint images and extracts pairs of the skeleton points. A skeleton point is a point that constitutes a skeleton.
The correspondence relationship modification means makes an examiner modify correspondence relationship of the points which forms a pair. The modification of the correspondence relationship of the points includes addition of correspondence relationship which makes points not forming a pair into a pair, deletion of correspondence relationship of the points forming a pair, and modification which moves the position of at least one of the points forming a pair.
The image distortion modification means modifies distortion of one fingerprint image based on positions of feature points forming a pair and skeleton points forming a pair. The skeleton point may use all of the skeleton points, or may use, for example, the skeleton points selected at a predetermined interval on the skeletons of one fingerprint and the corresponding skeleton points of the other fingerprint image. The image distortion modification means defines a vector the starting point of which is a position of a point of a pair on one image and the end point of which is a position of the other point of the pair on the other image, and, based on the vector, modifies image data and skeleton data around the point, on the fingerprint to be modified.
The image display means overlays one image and other modified image and displays them.